Sulfuric acid is utilized in large quantity in various kinds of industries with a grade (at a purity or a concentration) determined according to purposes of the industries. In the case that such sulfuric acids are spent, various methods for recycling the sulfuric acids have been proposed (in Japanese Patent Application Publication Laid-Open Nos. 9-315806, and 2002-68715, and so on), and, currently, the following processes are commonly performed.
(1) Wastewater treatment (or neutralization treatment) is performed thereon in a generating plant.
(2) A specialized processor picks up (or collects) spent sulfuric acids and performs wastewater treatment thereon.
(3) Spent sulfuric acids are utilized in a plant as a neutralizer.
(4) Sulfuric waste liquid is recycled (or subjected to distillation purification) in a plant or by a specialized recycler.
(5) Spent sulfuric acid is taken over to a specialized recycler, and utilized as a material of alum (or aluminium sulfate) (incidentally, in this case, the usage rate of spent sulfuric acid is equal to or less than 10%).
(6) Spent sulfuric acid is collected by a specialized recycler, and is resold to other users by mixing with virgin sulfuric acid.
In the above case (1), sulfuric acid itself is a strongly acidic powerful drug and therefore cannot be discarded as it is, and neutralization treatment by using a large amount of neutralizer, such as alkali (for example, slaked lime or the like), or coagulation-sedimentation treatment or hydrogen peroxide removal treatment by using various wastewater treatment agents (for instance, inorganic coagulant, polymer coagulant, or hydrogen peroxide decomposition accelerator) is required, and resultantly a large amount of sludge and wastewater are to be generated. Regarding the above case (2), it is the same with the case (1), except that the treatment is performed on the spent sulfuric acids outside a plant. In the above case (3), a demand for using the spent sulfuric acid as the neutralizer is small, and consequently, the same treatment as required in the case (1) is needed for treating a surplus part of the spent sulfuric. In the case (4), a huge plant and energy are needed for recycling the spent sulfuric acid, and the recycling thereof is largely affected by the location (or site condition) of the plant, so that a target area therefor is limited only to a partial restricted area. Moreover, the provision of recycling equipment in a generating plant is fraught with many problems in energy, cost, site area, safety, and handling thereof and thus impractical and hardly in common use. In the above case (5), waste sulfuric acid is seldom treated as a valuable resource, and usually, the waste sulfuric acid is taken over at charge or sold at a low price (several yen per kilograms). The waste sulfuric acid is transported by way of a path from a discharge source to a user through a specialized recycler, so that a transportation distance is elongated, and similarly with the above case (4), the utilization is largely affected by the location (or site condition). Similarly, in the above case (6), the spent sulfuric acid is transported by way of a path from a discharge source to a user through a specialized recycler. Thus, the transportation distance is elongated and the mixture is susceptible to the location (or site condition).
As described above, according to the prior art, the recycling of spent sulfuric acid is susceptible to the location of a specialized recycler that practices collection/takeover of spent sulfuric acid. Therefore, generally, in a case where no specialized recyclers are present in the vicinity of an own plant, wastewater treatment is performed in the own plant, and the recycling of spent sulfuric acid has not fully been established. According to the prior art, an added-value of spent sulfuric acid is low, and accordingly, wastewater treatment is usually performed by the above process (1) or (2) in general. Alternatively, the spent sulfuric acid is treated by being delivered to outsiders for value. However, it is obvious from the viewpoint of environment protection countermeasures that environmental destruction is increasingly promoted by repeating a treatment process, which is seen in the above case (1) or (2) and uses a large amount of chemicals to thereby generate large amounts of sludge and wastewater.
Thus, there has been an increasing demand for developing techniques related to a method of recycling spent sulfuric acid, and each office advances development directed to simplification and energy-saving of the method and equipment for performing the method. For example, a method of introducing sulfurous acid to spent sulfuric acid thereby to decompose a residual hydrogen peroxide solution and to generate sulfuric acid (see Japanese Patent Application Publication Laid-Open No. 9-135806) and a method of supplying spent sulfuric acid to an electrolytic cell constituted by anion-exchange membrane and cation-exchange membrane thereby to concentrate sulfuric acid and to generate oxidant (see Japanese Patent Application Publication Laid-Open No. 5-139707) have been studied in each office that undertakes various efforts directed to scaling-down of a plant and to energy-saving. However, these techniques have faced many problems in cost and so on. Thus, it cannot be said that a practical method of recycling spent sulfuric acid has fully been established.
An object of the invention is to provide an industrially advantageous new system featuring that spent sulfuric acid is effectively utilized at a place other than a discharge source regardless of geographical conditions.
Further, another object of the invention is to provide a system which is able to recycle spent sulfuric acid, without being equipped with a large-scale facility, at low cost by supplying the spent sulfuric acid to an appropriate user according to the necessary purity and amount thereof and by recycling the supplied acid.
Furthermore, further another object of the invention is to provide an industrially advantageous recycling method featuring that sulfuric waste liquid discharged from a certain office is effectively utilized by mixing the waste liquid with at least one of a hydrogen peroxide solution and water to thereby add a value to the sulfuric waste liquid, and to provide value-added recycled sulfuric acid.
Moreover, still another object of the invention is to provide a method enabled to recycle spent sulfuric acid at low cost without setting up a large-scale facility.